1. Field of the Invention
The present invention relates generally to composite aerogels and, more particularly, to an electrically conductive, mesoporous architecture that improves access of a catalyst to a reactant thereby leading to enhanced catalytic activity.
2. Background of the Invention
High-surface-area carbon blacks serve as effective solid supports to disperse nanoscale noble metals for use as catalysts (e.g., for hydrogenation or dehydrogenation reactions) and electrocatalysts for oxidation and reduction reactions in fuel cells, and to thereby increase efficiency and reduce costs. In fuel cells, despite high surface areas of the electron-conducting carbon support and effective dispersion of the Pt electrocatalysts, self-agglomeration of the carbon particles within practical electrode structures limits the approach of fuel and oxidant to the active sites, such that all of the electrocatalyst in a fuel-cell electrode cannot be accessed.
This lack of accessibility, also known as the xe2x80x9chidden Pt problem,xe2x80x9d is evidenced by the fact that current densities in fuel cells do not scale with Pt loadings. Higher weight loadings of noble metal catalysts are then needed in order to achieve complete oxidation of the fuel (i.e., to provide full fuel utilization). This problem may be further compounded by the use of polymeric binders to incorporate the carbon-supported catalytic powder into the desired electrode geometry for the practical power source.
Previous attempts have been made to create conductive carbon-silica composites derived from sol-gel chemistry. These composites were dried under ambient conditions to produce xerogel thin films or monoliths. Composite xerogels suffer certain disadvantages, which are discussed below.
In accordance with the present invention, an electrically conductive composite is provided which affords a number of advantages over previous composites used in fuel cells. For example, as compared to xerogel composites, the electrically conductive composites of the invention are considerably more porous than xerogel composites and provide a continuous transverse and longitudinal porous path.
The electrically conductive composite of the invention comprises a nanoscopic Pt electrocatalyst, a carbon black electron-conducting support, and silica aerogel. The electrically conductive composite exhibits oxidation activity from 0.5 to 100 mA/mg of Pt. In a preferred implementation of the present invention, the carbon support comprises Vulcan carbon. In yet another preferred implementation, the nanoscopic Pt electrocatalyst comprises colloidal Pt.
In accordance with yet another aspect of the present invention, a method of forming an electrically conductive composite is provided. The method includes the initial step of providing a desired amount of catalyst-modified carbon, which is then infused with an about-to-gel colloidal silica sol. Preferably, the amount of sol used is just enough to cover the bed of catalyst-modified carbon. The sol is then allowed to permeate the catalyst-modified carbon. A wet composite gel is then formed which comprises the catalyst-modified carbon and the colloidal silica. The wet composite gel is then dried supercritically (under conditions relevant to a specific pore fluid filling) to form a composite aerogel.
In a preferred method of the present invention, the catalyst-modified carbon comprises VULCAN carbon. The method of the present invention may be used with any pre-formed metal-modified carbon powder, preferably nanoscale-Pt-modified carbon. In an advantageous implementation of the method of the present invention, the method further comprises the step of using argon atmosphere annealing after the composite is formed, which increases Pt particle size to approximately 2-3 nm. Preferably, the annealing step is carried out at approximately 900xc2x0 C.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.